With the four walls all framed, it is time to sheath the outside. Since you took so much care in making the studs regularly spaced the sheathing should fly by. You will want to swap out the 16D nails in the nailer for 8D nails at this point. Exterior grade OSB has an inner and outer side. This makes more of a difference when roofing as the outer side is textured to improve traction when walking on it. However, the other feature of the outer side is a painted grid of 16" and 24" spaced lines to allow you to hit the studs underneath when nailing. Make sure this side is out and you will save yourself some headaches.
The easiest way to sheath your walls is to sheathe right over the small door and window openings. Once they are nailed in place someone working from the inside can drill holes at the corners, come around to the outside, and using a jigsaw or reciprocating saw connect these holes to open up the windows and doors. Presto chango!
While sheathing, you will need to put nails in every 6-8" or so on the edges and on the interior studs. Check your local codes. With 24" stud spacing there will only be one interior stud. The hardest part is to hit the studs when your studs are off of the painted grid on the OSB. You can also have a spotter inside to tell you if you are missing the studs. This is no big deal if you are going to insulate and close up the wall cavities, but if you plan on leaving the interior walls open you will want to pound those nails back out so you don't cut yourself later down the road. Your spotter can do that while you nail.
Since my walls are about 8' 5" tall a single sheet of 4'x8' OSB will not cover the whole wall. My approach was to start the panel at the sill plate and to run the panel vertically, leaving the last 5 inches or so open. The goal here is to stiffen up the walls with the sheathing so you can put on the roof. If you are so inclined you can have someone following the nailer with strips of OSB to fill these gaps or wait for later. However, they can only do the "flat" walls since on the gable ends the sheathing will need to extend up to the peak of the roof trusses.
See the pics below to get an idea of what the garage will look like with this lower course of OSB on.
Another thing to note is that the South wall on my garage has no windows. When you build within a certain distance of the property line (3 feet) in Minneapolis and elsewhere you cannot have windows or doors in the wall. Additionally, I had to cover the exterior studs with fire-rated exterior drywall rather than OSB on the wall facing my neighbor's property. The drywall also had to extend all the way to the lower surface of the roof deck. The interior wall on this side also had to be fire-rated drywall in order to prevent a fire in my garage from spreading to the neighbor's. Check your local code, and read this document for more information.
With the lower 8' of the garage sheathed, it is time to put on the roof trusses.